Because right now those fired employees still _need_ to work, so they apply somewhere else. Even the most incompetent eventually get past the filters and gum up someone else's operation. So we have a cycle in which we harden the filters (e.g. day-long interviews for programming jobs) which makes life worse for everyone involved.

If those bad employees could self select the "good enough" lifestyle provided by a basic income, we could reduce all the overheads associated with trying to protect workplaces from them.